Kinetics of thermalized luminescence of a single quantum dot at room temperature

We have developed a theory of transient secondary emission of a single quantum dot from the lowest energy states of electron-hole pairs. We consider a process in which laser pulses excite a certain highenergy state of electron-hole pairs of a quantum dot at room temperature, with the electronic subsystem then relaxing to low-energy states and photons being emitted. Therefore, the investigated secondary emission process is thermalized luminescence. For definiteness, the developed model takes into account two states of electron-hole pairs that contribute to the luminescence. We have analyzed the dependence of the secondary emission signal on the energy gap between these states, the value of which is determined by the quantum dot size. In terms of the Pauli master kinetic equation, an analytical expression for the time dependent signal of the thermalized luminescence has been obtained. We show that, as the spectral width of the exciting laser pulse tends to zero, this expression yields the signal of stationary luminescence.     

Optical phase-space reconstruction of mirror motion at the attometer level

We describe an experiment in which the quadratures of the position of an harmonically-bound mirror are observed at the attometer level. We have studied the Brownian motion of the mirror, both in the free regime and in the cold-damped regime when an external viscous force is applied by radiation pressure. We have also studied the thermal-noise squeezing when the external force is parametrically modulated. We have observed both the % theoretical limit of squeezing at low gain and the parametric oscillation of the mirror for a large gain. Received 9 July 2002 Published online 29 October 2002 RID="a" ID="a"e-mail: heidmann@spectro.jussieu.fr RID="b" ID="b"Unité mixte de recherche du Centre National de la Recherche Scientifique, de l'école Normale Supérieure et de l'Université Pierre et Marie Curie. Website: www.spectro.jussieu.fr/Mesure     

Kinetics of resonance luminescence of a single quantum dot at room temperature

We have developed a theory of transient resonance luminescence of a single quantum dot from the lowest energy states of electron-hole pairs. We consider a process in which laser pulses directly excite photonemitting states of electron-hole pairs of the quantum dot at room temperature. For definiteness, the model under the development takes into account two states of electron-hole pairs that contribute to luminescence. We have analyzed the dependence of the secondary emission process on the energy gap between these states, the value of which is determined by the quantum dot size. In terms of the Pauli master kinetic equation, an analytical expression for the time-dependent signal of the resonance luminescence has been obtained. We show that, as the spectral width of the exciting laser pulse tends to zero, this expression yields the signal of stationary luminescence.     

Spatial correlation of an electron and hole in quasi-two-dimensional electronic system in a strong magnetic field and its relationship to the light-scattering tensor

The spatial correlation of light-generated electrons and holes in a quasi-two-dimensional electron gas in a strong magnetic field is investigated in an approximation linear in the intensity of the exciting light. The correlation is due to the interaction of the electrons and holes with longitudinal optical (LO) phonons. The theory permits calculating, on the basis of a special diagrammatic technique, two distribution functions of an electron-hole pair with respect to the distance between the electron and the hole after the emission of N phonons: first, the function determining the total number of pairs which have emitted N phonons and, second, the function related to the rank-4 light-scattering tensor in interband resonance Raman scattering of light. A special feature of the system is that the electron and hole energy levels are discrete. The calculation is performed for a square quantum well with infinitely high barriers. The distribution function and the total number of electron-hole pairs before the emission of phonons as well as the distribution function corresponding to two-phonon resonance Raman scattering are calculated. The theory predicts the appearance of several close-lying peaks in the excitation spectrum under resonance conditions. The number of peaks is related to the number of the Landau level participating in the optical transition. The distance between peaks is determined by the electron-phonon coupling constant. Far from resonance there is one peak, which is much weaker than the peaks obtained under resonance conditions. Zh. éksp. Teor. Fiz. 111, 2194–2214 (June 1997)     

Photoinduced superstructure in the low-temperature phase of a Peierls system

This paper is a theoretical study of the properties of the low-temperature phase of a Peierls system when nonequilibrium electron-hole pairs are excited in the phase. A microscopic theory is developed to show that at low temperatures a spatially nonuniform periodic structure with a modulated band gap forms in the thermodynamically nonequilibrium system considered. The critical temperature of formation of such a superstructure, the critical electron-hole pair concentration, the spatial period, and the percentage modulation are calculated. Zh. éksp. Teor. Fiz. 115, 1297–1314 (April 1999)     

Picosecond luminescence spectroscopy of highly excited GaAs

Time resolved luminescence of highly excited GaAs is studied using a streak camera. We observe the Mott transition from the electron-hole plasma to the excitonic state. This transition is smooth and does not show a phase separation. The plasmon sideband of the electron-hole plasma emission is identified.     

Cooling of high density electron-hole plasma

We generalize for high density electron-hole plasma, the previous theories [12] of temperature cooling of non-equilibrium hot plasma. Especially we take into account the cooling by emission of mixed longitudinal optical phonon and plasmon modes, these quasiparticles described by a non-equilibrium distribution function. We show that a strong slowing of the plasma cooling occurs, at high electron-hole density. We calculate for CdSe the temperature kinetics of plasma created by Yag laser (pulse duration 30 ps).     

Induced absorption of light in the magnetoelectric-photo effect in InSb: Saturation at low temperatures

An induced absorption of photons observed in an electron-hole plasma of InSb in the transverse configuration (JH) of the current density J and the magnetic field H has been demonstrated to saturate at high values of J×H at low temperatures. Nevertheless, strong emission of photons having a central wavelength corresponding to the energy gap is still observed at 80 K, suggesting generation of excess electron-hole pairs by the action of the J×H force. The mechanism of the induced absorption is also discussed.     

Picosecond buildup and relaxation of intense stimulated emission in GaAs

In support of the idea developed previously based on circumstantial evidence, we have found that stimulated emission emerges in GaAs and its intensity increases with a picosecond delay relative to the front of powerful picosecond optical pumping that produced a dense electron-hole plasma. The emission intensity relaxes with decreasing pumping with a characteristic time of ～10 ps. We have derived the dependences of the delay time, the relaxation time, and the duration of the picosecond emission pulse on its photon energy. The estimates based on the fact that the relaxation of emission is determined by electron-hole plasma cooling correspond to the measured relaxation time.     

Observation of bound exciton and distant pair resonances in amorphous phosphorus

For the first time in an amorphous semiconductor magnetic resonance of a bound exciton is reported. ODMR investigations of amorphous phosphorous show that the luminescence consists of two bands, a low energy emission which is consistent with triple exciton recombination at IVAPs and a high energy emission due to electron-hole recombination at distant IVAPs. The triplet exciton has g = 2.13 ± 0.05, and the distant pair resonance occurs at g = 2.0 ± 0.02.     

Rapid formation kinetics of a long-lived electron-hole drop in Ge under pulsed excitation

The buildup of a large long-lived electron-hole drop in stressed Ge is studied using its Alfvén wave resonances. This microwave method permits rapid time resolution of the formation process after a short intense (100 nsec, 7 W) laser excitation pulse. A delay of about 1 μsec is observed before the drop radius rapidly increases from zero to ～ 100 μm within the next μsec. This is consistent with the interpretation that the photo-produced electron-hole pairs are quickly accelerated to the strain-induced potential minimum which is well within the crystal.     

Letter to the editor

Experimental results on the recovery spectra of low temperature electron irradiated pure Ag and AgZn alloys in the concentration range 0.018 to 30 at.% of zinc are presented. Their comparison leads to consider two distinct concentration domains. In the low concentration domain he classical model of Corbett, Smith and Walker applies with some modifications. The resistivity ecovery is governed by the combinations and annihilations of point defects. It must be noted hat even in dilute alloys, new kinds of point defects (such as mixed interstitials) arise from the presence of solute atoms. In the high concentration domain, the resistivity recovery is dominated by the setting in of short-range order as a result of the point defect migration: interstitia migration in stage II and, mainly, vacancy migration in stage III. The defect migration, free in he pure metal, is influenced, in the concentrated alloys, by the local environment. The proposed model consistently allows the interpretation of the existing results, particularly from electron microscopy.

Nous présentons des spectres d'évolution de la résistivité électrique obtenus lors de recuits aprés Irradiation électronique à basse température, dans I'argent pur et dans des alliages AgZn dont a concentration varie de 0,018 à 30% at. de zinc. La comparaison des résultats amène à considérer deux domaines de concentration. Dans le domaine des faibles concentrations, le modèle classique de Corbett, Smith et Walker avec quelques modifications explique bien ces spectres. Le recuit de la résistivité est régi par les combinaisons et annihilations des défauts ponctuels. Il importe de ne pas perdre de vue que la présence d'impuretés měme en faible concentration peut induire de nouvelles expèces de défauts (interstitiels mixtes, par exemple). Aux concen-rations plus élevées, le recuit de la résistivité est dominé par la mise en ordre à courte distance qui résulte de la migration des défauts; interstitiels dans I'étape II et, surtout, lacunes dans l'étape III. La migration des défauts, libre dans le métal pur ou les alliages dilués, est influencée par I'environnement local dans le cas des alliages concentrés. Le modèle proposé permet une nterprétation cohérente des résultats existants, en particulier de microscopie électronique.     

Luminescence line shape of free excitons in pure Ge

We report luminescence experiments performed in pure Ge at low temperature. Taking into account the splitting of the ground state of free excitons in this material, we show that their emission line shape is subject to a Gaussian broadening which seems to be due to the phonon lifetime. From this study, we deduce new values of the binding energy of electron-hole drops in Ge and Si which are respectively -2 and -5.6meV.     

Luminescence of a quasi-two-dimensional electron-hole liquid and excitonic molecules in Si/SiGe/Si heterostructures upon two-electron transitions

The low-temperature photoluminescence of Si/Si_0.91Ge_0.09/Si heterostructures in the near-infrared and visible spectral ranges is investigated. For the structure in which the barrier in the conduction band formed by the SiGe layer is transparent to electron tunneling, the broad luminescence line observed in the visible range is analyzed by comparing its shape with the numerical convolution of the spectrum of near-infrared recombination radiation originating from the electron-hole liquid. The comparison demonstrates that, at high excitation levels, the visible-range emission is caused by two-electron transitions in a quasi-two-dimensional spatially direct electron-hole liquid. Furthermore, the combined analysis of the photoluminescence spectra in the near-infrared and visible ranges yields the binding energy of a quasi-two-dimensional free biexciton in the SiGe layer of these heterostructures. In the structures with a wide SiGe layer that is not tunneling-transparent to electrons, a spatially indirect (dipolar) electron-hole liquid is observed.     

Effect of deep impurity of recombination lifetime of a large electron-hole drop in strained germanium

Lifetime shortening of a large electron-hole drop in zinc-doped Ge is reported by using time-resolved photoluminescence technique. The lifetime decreases remarkably with the concentration of a double acceptor Zn up to ∼10¹⁶ cm⁻³. This makes a striking contrast to the result by Chen et al. for ordinary electron-hole droplets in Ge doped with shallow impurities. Additional information from microwave Alfvén wave dimensional resonance of a large drop is also presented.     

Finite size effects on the optical transitions in quantum rings under a magnetic field

We present a theoretical study of the energy spectrum of single electron and hole states in quantum dots of annular geometry under a high magnetic field along the ring axis in the frame of uncorrelated electron-hole theory. We predict the periodic disappearance of the optical emission of the electron-hole pair as the magnetic field increases, as a consequence of the finite height of the barriers. The model has been applied to semiconductor rings of various internal and external radii, giving as limiting cases the disk and antidot.     

On the lifetime of certain excited states of chemisorbed atoms

For the determination of the ESD (electron stimulated desorption) cross section, the deexcitation process of the adsorbate plays an important role. In this paper we calculate the decay rate of the neutral adsorbate which is excited to the antibonding state, taking into account the electron-hole excitations in the metal. A similar technique which was used by Nozieres and Dominicis for the X-ray emission problem is applied for the summation of the diagram expansion. The lifetime of the antibonding state is estimated by the use of a simple model for the electron-hole excitation spectrum.     

Quantization of solitons and the restricted sine-Gordon model

We show how to compute form factors, matrix elements of local fields, in the restricted sine-Gordon model, at the reflectionless points, by quantizing solitons. We introduce (quantum) separated variables in which the Hamiltonians are expressed in terms of (quantum) τ-functions. We explicitly describe the soliton wave functions, and we explain how the restriction is related to an unusual hermitian structure. We also present a semi-classical analysis which enlightens the fact that the restricted sine-Gordon model corresponds to an analytical continuation of the sine-Gordon model, intermediate between sine-Gordon and KdV. Laboratoire associé au CNRS. Laboratoire de la Direction des Sciences de la Matière du Commissariat à l'Energie Atomique.     

Laserlike vibrational instability in rectifying molecular conductors

We study the damping of molecular vibrations due to electron-hole pair excitations in donor-acceptor (D-A) type molecular rectifiers. At finite voltage additional nonequilibrium electron-hole pair excitations involving both electrodes become possible, and contribute to the stimulated emission and absorption of phonons. We point out a generic mechanism for D-A molecules, where the stimulated emission can dominate beyond a certain voltage due to the inverted position of the D and A quantum resonances. This leads to current-driven amplification (negative damping) of the phonons similar to laser action. We investigate the effect in realistic molecular rectifier structures using first-principles calculations.     

Quadratic response of the intermediate state to a transverse alternating magnetic field

The response or susceptibility of the intermediate state in a transverse cylinder of indium to a small alternating magnetic field perpendicular to both the static magnetic field and the axis of the cylinder is studied as a function of frequency, size, resistivity and temperature. The response is quadratic as a second harmonic signal is generated in a pickup coil axial to the static magnetic field. At sufficiently low frequencies the quasi-static limit is attained. At intermediate frequencies the signal is eddy-current damped, and at higher frequencies the signal is dependent on the structure of the intermediate state.

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Zusammenfassung Die Suszeptibilit?t des intermedi?ren Zustandes eines transversalen Zylinders von Indium wurde mit Hilfe eines kleinen magnetischen Wechselfeldes (senkrecht zum statischen Magnetfeld und zur Zylinderachse) als Funktion der Frequenz, der Dimensionen, des Widerstandes und der Temperatur untersucht. Ein quadratisches Verhalten wird gefunden, da ein Signal doppelter Frequenz in einer Pickup-Spule axial zum statischen Magnetfeld erzeugt wird. Bei genügend niedriger Frequenz wird der quasistatische Grenzfall erreicht. Bei mittleren Frequenzen ist das Signal durch Wirbelstr?me ged?mpft, und bei hohen Frequenzen h?ngt das Signal von der Struktur des intermedi?ren Zustandes ab.

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Résumé La réponse, ou susceptibilité, dans l'état intermédiaire d'un cylindre d'indium à un champ magnétique alternatif perpendiculaire à la fin au champ statique et à l'axe cylindre est étudiée en fonction de la fréquence, des dimensions, de la résistivité et de la température. La réponse est quadratique puisque un signal d'harmonique deux est engendré dans une bobine de détection parallèle au champ statique. A fréquences suffisamment basses la limite quasi-statique est atteinte. A fréquences intermédiaires le signal est amorti par Courants de Foucault, et à hautes fréquences le signal dépend de la structure de l'état intermédiaire.